The present invention relates to a time division multiplex system, and particularly to such a system useful as a private automatic branch telephone exchange.
At the present time there is an increasing demand by administrative, business, and other types of organizations for private automatic branch exchanges to provide efficient means of internal communication as well as access to public telephone networks. The exchanges are expected to accommodate a wide-range of service classes including special features required by the customer. Substantial progress in the field of private branch exchanges (PBX's) has been attained, the trend being towards automatic exchanges (PABX's) which allow direct internal and outgoing dialing, requiring the operator to deal only with incoming calls and special services.
A PABX is characterized by the organization of the control and switching network. The control may be either step-by-step, with direct set-up of calls according to subscriber's dialing; or by common-control, where the dialed number is stored and the call process according to the classes of services available between the calling and called subscribers.
The control section of a modern exchange is usually made of solid-state electronic components. The switching network of the modern PABX may, however be either electromechanical or electronic.
Electromechanical switching networks (relay, crossbar, etc.) all belong to a class called space division (SDS) networks, where the connections between subscribers are established by means of a physical link between them at the time of the call. Electronic switching networks, however, are of two classes, namely: space division (SDS) networks, utilizing stages of solid-state switching matrices; or time division (TDM) networks.
Time division switching is based on the technique of sampling a subscriber's speech and applying the samples in an appropriate time-slot to a common transmission highway. The sampling rate is always equal to or more than twice the highest frequency in the speech frequency band. Connections between two subscribers are established by opening their respective gates to the highway at the proper time-slot. The sampled speech of each subscriber is reconstructed to its analog form and is then received by the other subscribers.
The conventional time division multiplex system includes a plurality of subscriber links each for connection to a subscriber line, the subscriber links including a switching network having an in-gate and an out-gate for each subscriber line for accessing it to a common transmission highway, and a common control controlling the in-gate to sequentially connect a calling subscriber link to the common transmission highway in the time-slot of the calling subscriber.
In order to set up a conference link between subscribers in the conventional time division multiplex system, the conferees are directed through the switching network to an analog-multiport circuit in which the speech signals are combined. As each subscriber is connected to the multiport circuit, he receives the speech signals from all the conferees except his own. Adding of the signals may be accomplished either in space or time.
Adding the signals directly in a conventional time division multiplex exchange, by adding the appropriate time-slots, requires the writing of all the conferee addresses in each line of the memory. Such a system therefore requires a memory having a large capacity. In addition, if the switching network has sample and hold circuits, the conferee signals cannot be added in the same frame, as they will be distorted. That is, a sample and hold circuit, whether of the pulse lengthener type or of the box-car type, can receive information in one frame from only one source without causing distortion. Thus, such a system does not permit the use of sample and hold circuits, but rather requires the use of a more complicated speech reconstruction network.
In addition, the conventional time division multiplex system usually provides for operator's intercept by having the operator disconnect one of the connected lines and connect instead the operator's line. One disadvantage of this arrangement is that it requires an additional manipulation on the part of the operator. Another disadvantage is that the disconnected party may not be aware that the disconnection is only temporary and may therefore on-hook, i.e. hang-up.